Welcome to the forums!!
I have not used the cantelever mount on the 930. I have the 28" barrel on mine and I have shot slugs with it. It was saticfactory just using the bead sight. I have used cantelever mounts on a Remington 870 and found it to be very accurate, I have no reason to think the Mossberg would not be of the same level of performance. (I do wish they would offer a 20" or 22" barrel though)
I shot a turkey two seasons ago with the 930 and it did an awesome job at about 25 years using the same 28" barrel, Primos Tite Wad choke and Winchester Heavy turkey loads 3" #5's.
I know that doesn't specifically answer your questions but based on my experience, I think you will be happy with the results.